The commercialization of photovoltaic devices, devices capable of directly converting radiant energy into electrical energy, is dependent on technological advances that contribute to improving the conversion efficiencies and stability of such devices. Several different approaches have been taken toward the development of photovoltaic devices, these approaches being directed towards the development of crystalline, polycrystalline and amorphous semiconducting materials for photovoltaic uses. Each type of material has presented unique technical difficulties that need to be overcome.
Among the polycrystalline semiconducting materials that are preferred for photovoltaic devices are p-type Class IIB-VIA semiconductor compounds that may be provided as thin films. Cadmium telluride and mercury cadmium telluride are examples of such compounds. However, it is difficult to establish ohmic contact with such compounds because these compounds do not readily form into layers having very low resistivity, and tunnelling junctions are required for ohmic contacts. For use in photovoltaic devices a low resistance ohmic contact is necessary for efficient energy conversion.
To make an ohmic contact with a p-type Class IIB-VIA semiconductor and an electrically conductive metal, the p-type Class IIB-VIA semiconductor is etched with an acidic solution, then a suitable metal is deposited on the etched semiconductor surface. Acidic solutions commonly used for etching include H.sub.2 SO.sub.4 +K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 O; HNO.sub.3 +K.sub.2 Cr.sub.2 O.sub.7 +H.sub.2 O; HNO.sub.3 +K.sub.2 Cr.sub.2 O.sub.7 +AgNO.sub.3 +H.sub.2 O; HF+H.sub.2 O.sub.2 +H.sub.2 O; HF+HNO.sub.3 +H.sub.2 O; HNO.sub.3 +H.sub.2 O and bromine methanol.
It is believed that the acidic etch produces a surface rich in Class VIA element, which improves the electrical contact between the metal to be deposited and the p-type Class IIB-VIA semiconductor material. Such acidic etchings have been used to improve the ohmic contact between the metal to be deposited and low resistivity, single crystal p-type semiconductors.
However, when the Class IIB-VIA semiconductor is provided as a polycrystalline thin film, generally having a thickness of less than 10 micrometers, then other factors become significant. These thin films possess relatively high resistivities, that is a resistivity greater than about 100 ohm-centimeters as compared to much lower values in a single crystal counterpart material. Typical resistivity values for a thin film of cadmium telluride, for example, is about 10.sup.4 ohm-cm. Strong acidic etches used for single crystal materials are too uncontrolled and abrasive for use on thin films, with the exception of very dilute bromine methanol, on the order of approximately 0.1 percent bromine in methanol. Applicant has disclosed an improved method of forming ohmic contacts between thin film p-type Class IIB-VIA semiconductor compounds and a conductive metal layer in U.S. Pat. No. 4,456,630, which disclosure is incorporated herein by reference. In this method, a thin film of a p-type Class IIB-VIA semiconductor compound is etched with an acidic solution, preferably an oxidizing acidic solution, then treated with a strong basic solution and finally a conductive metal layer is deposited on the etched and treated surface of the semiconductor.
Whereas these earlier-mentioned methods sought to improve the ohmic contact between a conductive metal layer and a p-type semiconductor compound, little consideration was given to the economic feasibility of these improvements. As an example, the most preferred conductive metal layer which forms an ohmic contact to these semiconductor compounds is gold. Such use of gold is not a viable alternative in the commercial production of low cost photovoltaic devices.
What is needed in the area of forming ohmic contacts between p-type thin films and conductive metals is a further improvement to produce a stable and economically feasible ohmic contact.
It is therefore one object of the present invention to provide stable, economically feasible ohmic contacts between p-type semiconductor thin films and conductive metals.
It is a further object of the present invention to provide a method for forming such ohmic contacts.
These and other objects of the present invention will become apparent to those skilled in the art in the following description of the invention and in the appended claims.